pkg/osutil: fix for getrlimit on FreeBSD
maxFD did not build on FreeBSD because syscall.Getrlimit returns an
int64 instead of a uint64.
So we convert the result to a uint64.
Credits to @SamWhited on github for the report + fix.
Fixes #925
Change-Id: I15ab682f2f80463279d24ece21a45c23f9a431e6